1. Field of the Invention
The field of the invention is that of optical systems, more particularly that of optical astrometry systems.
2. Description of the Prior Art
Astrometry is a well known technique consisting in simultaneously observing, along two sightlines, two “fields of view” containing stars, determining the positions of the stars in each field of view, and measuring the real positions of the stars by comparing their positions within their respective fields of view. Obtaining a given measurement accuracy demands at least equivalent stability on each sightline.
There are two well known solutions for implementing the above technique.
A first solution consists in using a single telescope having two sightlines defined by a first mirror, consisting of two portions angularly separated by half the angle between the two sightlines, and combining the two light beams at the first mirror. In this first solution, as implemented on the HIPPARCOS sky mapper satellite, the required stability of the two sightlines is reflected in a demand for angular stability in respect of only the two portions of the first mirror. However, because there is only one focal plane, identifying the field of view pertaining to an observed star must be based a priori on an existing catalog, which is a major handicap in the case of measuring the positions of new stars.
A second solution consists in using two telescopes each having a sightline and the two beams of which are combined in a single focal plane after forming an intermediate image at an intermediate focal plane. By placing a mask in the intermediate focal plane of one of the two telescopes, it is possible, after combining the two beams, to identify in the single focal plane the field of view pertaining to an observed star, thanks to a lateral shift of the two fields equal to a detection column. This solution can measure the positions of new stars, but requires that the sum of the stabilities of each component of each telescope be lower than the required measurement accuracy, which is particularly difficult to achieve, especially if the measurements are effected by telescopes on board a satellite rotating about a heavenly body, such as the Earth.
An object of the invention is therefore to improve on this situation.